High-power lasers such as excimer lasers, YAG lasers, and the like are presently becoming widespread not only in research applications, but in industrial applications as well. These fields of industrial utilization are expanding not only into general materials processing, but also into the medical and semiconductor fields.
When materials processing is performed using an excimer laser or similar beam, the beam is transformed in a linear cross-sectional dimension by means of an optical system, with scanning in the latitudinal direction (width direction).
The longitudinal direction and the latitudinal direction of the beam cross-section are each severally divided by a cylindrical lens to obtain a high degree of uniformity in the longitudinal direction and latitudinal direction of the beam pattern when the laser beam is transformed to a linear cross-sectional configuration.
FIG. 7 is a conceptual diagram of an apparatus operating according to the conventional laser annealing method (refer to Japanese Patent Application Laid-open No. H8-195357).
The apparatus 1 depicted in the diagram is designed such that a beam 2 from a YAG laser (not shown) is divided into four parts in a vertical direction by a lens group 3 comprising four cylindrical lenses 3a-3d, further subdivided into seven parts in a horizontal direction by a lens group 4 comprising seven cylindrical lenses 4a-4g, and combined by a paired lens 7 comprising a pair of cylindrical lenses 7a and 7b disposed orthogonally to the generatrix, yielding a beam pattern whose light intensity is uniform in the longitudinal direction and the latitudinal direction. (Lens groups 3 and 7 comprise a homogenizer 15.) A beam 8 whose light intensity is uniformized is deflected by a reflecting mirror 9 towards a sample 10 and focused by a cylindrical lens 11 such that the sample 10 placed on a translation stage 13 that moves in the direction of arrow 12 is irradiated by a linear beam 14.
Due to the occurrence of transverse expansion when a YAG laser is used in this arrangement, the shape of the beam in the direction of the minor axis may correspond to that of a Gaussian beam.
FIG. 8A is a block diagram of a pulse YAG laser (hereafter referred to as “YAG laser”) as a laser light source, and FIG. 8B is an arrow view of a laser amplifier on the output side of the YAG laser depicted in FIG. 8A in the direction of arrow A. FIG. 9A is a diagram depicting the beam pattern of the YAG laser; FIG. 9B is an intensity distribution diagram of the beam pattern along the line 9B—9B depicted in FIG. 9A; and FIG. 9C is an intensity distribution diagram of the beam pattern along the line 9C—9C depicted in FIG. 9A. In FIGS. 9B and 9C, distance is plotted on the horizontal axis, and light intensity is plotted on the vertical axis.
It is apparent from FIG. 9B that the beam 24 of the YAG laser has a Gaussian distribution 24a, and from FIG. 9C that the light intensity thereof has large peaks 36 and 37 on both ends (the top and bottom ends in FIG. 9A).
This is because flash lamps 32 and 35 for excitation are disposed at both sides of NdYAG rods 31 and 34.
A description will now be given of the YAG laser depicted in FIGS. 8A and 8B.
The YAG laser 20 comprises an output laser oscillator 21 for oscillating a pulsed YAG laser, two laser amplifiers 22 and 23, and reflecting mirrors 25 and 26 for deflecting the path of the beam 24 from the output laser oscillator 1 and inputting the resultant beam into the laser amplifier 22 of the preceding stage.
The output laser oscillator 21 comprises a resonator comprising a total reflection mirror 27 and a diffusion (output) mirror 28, an NdYAG rod 29 disposed at the central axis of the resonator; and a flash lamp 30 for generating pulsed light flashes as excitation light arranged parallel with (in the y-axis) and beneath the NdYAG rod 29.
The laser amplifier 22 of the preceding stage comprises an NdYAG rod 31 disposed along the optical axis of the beam 24 from the reflecting mirror 26, and a flash lamp 32 arranged parallel with (in the y-axis) and beneath the NdYAG rod 31.
The later-described laser amplifier 23 comprises a NdYAG rod 34 disposed along the optical axis of the beam 33 arriving from the laser amplifier 22 of the preceding stage, and a flash lamp 35 arranged parallel with (in the y-axis) and beneath the NdYAG rod 34.
For this reason, the strong portions of the light intensity from the excitation light of the flash lamps 32 and 35 are superimposed on both ends of the beam 24 (depicted by the broken line) emitted from the YAG laser 20 and provided with the Gaussian distribution 24a, thus generating large peaks 36 and 37 as depicted in FIG. 9A on the upper and lower ends (in the y-axis) of the beam pattern 38.
A linear beam having large, streaked peaks on both latitudinal (in the direction of arrow 12) ends thereof in the manner depicted in FIG. 10 is radiated to the sample 10 when a beam 24 having large peaks 36 and 37 is directly transformed to a linear cross-sectional configuration with the aid of cylindrical lens groups 3-6 or a paired lens 7 such as those depicted in FIG. 7. A resulting problem is that the sample 10 undergoes ablation (a phenomenon in which scattering and surface roughening occur in areas within the portion of the sample 10 exposed to the beam 14 that are irradiated by the streaked-peaks, specifically, the longer ends). FIG. 10 is a diagram depicting the light intensity (along the line 10—10) of a linear cross-sectional beam applied to a sample 10 from a laser annealing apparatus as depicted in FIG. 7.